Displaying file blueprint fields values in block preview

What is the right way to display content from file blueprint in block preview?

For example: I take a default Gallery block and want to allow captions to be added and displayed under the images. I add a new field called caption to the appropriate file blueprint. Then I would like to adjust Gallery block preview in the panel to show captions, so I create a simple plugin where the template looks like this:

template: `
<div @dblclick="open">
	<ul>
		<template v-if="content.images.length === 0">
			<li class="empty"/>
			<li class="empty"/>
			<li class="empty"/>
		</template>
		<template v-else>
		<li v-for="image in content.images" :key="image.id">
			<figure>
				<img :src="image.url" :srcset="image.image.srcset" :alt="image.alt" />
				<figcaption>{{ image.caption }}</figcaption>
			</figure>
		</li>
		</template>
	</ul>
</div>
`

image.caption is empty, because image object does not contain blueprint fields values in this context (I guess?). Do I need to somehow load it first before using?